In general, in free radical polymerization (including copolymerization), for a given initiator-monomer system, an increase in the polymerization rate can be obtained by increasing the initiator concentration and/or the polymerization temperature. However, the increase in rate of polymerization is accompanied by a corresponding decrease in molecular weight of the polymer. Therefore, there is great interest in any process that allows one to increase the polymerization rate while maintaining, or even increasing, the molecular weight of the polymer produced.
There are a number of patents which describe a process for the polymerization of styrene in two or more stages and using a mixture of two or more free-radical initiators (e.g. see U.S. Pat. Nos. 2,656,334 and 2,907,756). The polymerization at each stage is conducted isothermally.
The disadvantage of these processes is that the polymerization time (or cycle time) is very long.
In British Pat. No. 1,243,197, Squire and Gammon describe a three component initiator system and a programmed heating cycle in which the temperature is continuously increased, whereby the cycle times can be reduced considerably. In Canadian Pat. No. 892,672, Squire and Gammon describe a process for the polymerization of vinyl aromatic monomers in the presence of two or more free-radical initiators under a programmed heating cycle. They also state that the molecular weight can be raised by adding small amounts of crosslinking agents such as divinyl benzene. In these patents, there is no indication that by using polyfunctional initiators, one can increase the molecular weight of the polymer. In fact, the use of a crosslinking agent (such as divinyl benzene) is advocated to increase the molecular weight of the product.
Belgian Pat No. 668,325 describes a process for suspension polymerization of styrene in two heating steps with the aid of an unsymmetrical diperester. Similarly, U.S. Pat. No. 2,698,863 describes the use of symmetrical diperesters for the polymerization of vinyl monomers at temperatures not exceeding 50.degree. C. Another U.S. Pat. No. 3,585,176 describes the use of unsymmetrical diperesters in two heating steps. Further, the unsymmetrical diperesters of the type disclosed in U.S. Pat. No. 3,585,176 (e.g. di-t-butyl dimethyldiperoxysuccinate) do not give high molecular weight polymer, even when used in conjunction with a programmed temperature cycle.
U.S. Pat. No. 3,817,965 discloses a process for the polymerization of vinyl compounds in suspension to yield high molecular weight polymers. In the process, the temperature of the suspension is increased rapidly from 95.degree. C to a temperature in the range of 100.degree.-150.degree. C and then the temperature is increased (more or less) linearly to a second higher temperature in the range of 120.degree.-160.degree. C. The molecular weight is primarily controlled by adjusting the rate of temperature increase.
In British Pats. Nos. 1,366,976 and 1,366,977, di-t-butylperoxyhexahydroterephthalate and di-t-butylperoxyhexahydroisophthalate are used as initiators in the polymerization of styrene, and higher molecular weight polystyrene is obtained, as compared to a benzoyl peroxide initiated system. Polymers of a given molecular weight can be obtained in a shorter time by increasing the initiator concentration.
Ivanchev, et al. [Vysokomol. Soyed., A11, (9), 2082 (1969) English translation in Polymer Sci. U.S.S.R. 11:9 (1969), A12, (2), 450 (1970) English translation Polymer Sci. U.S.S.R., 12:514 (1970] investigated the use of symmetrical diperoxides and unsymmetrical diperoxides in isothermal styrene polymerization. They reported that the unsymmetrical diperoxides were capable of giving much higher molecular weight polymer but the molecular weight distribution was bimodal at both low and high conversions. That is, a differential curve of molecular weight distribution showed two distinct peaks, e.g., see Polymer Sci. U.S.S.R. 12, 514 (1970) at page 517, FIG. 3. When symmetrical diperesters were used, Ivanchev et al. reported that the molecular weight distribution was unimodal, like the monoperesters but the maxima in the molecular weight distribution curve for the diperesters was lower than that obtained with monoperesters (e.g. t-butyl perbenzoate).
For commercial polymers (e.g. crystal polystyrene), a unimodal molecular weight distribution is required in order that the processing characteristics remain unaffected. Thus the prior art suggests that unsymmetrical diperoxides will not give an acceptable unimodal molecular weight distribution.
Bi- or tri-modal molecular weight distributions were also reported when triperoxides, which decompose by a stepwise first order mechanism, were used as initiators (see Ivanovich et al, Vysakamol. Soyed. A14 (5), 1027 (1972).
Thus Ivanovich et al. teach that symmetrical diperoxides do not give higher molecular weight polymer; that the unsymmetrical diperoxides do give higher molecular weight polymer but with a molecular weight distribution that is bimodal or, in some instances, trimodal.
A Noury product bulletin (5-103-2, Aug. 1973) states that 1,1 di-t-butylperoxy 3,3,5-trimethylcyclohexane gives a higher polymerization rate than does t-butyl perbenzoate and also higher molecular weight, in the isothermal polymerization of styrene at 100.degree. C and 110.degree. C.